Over the last decade or so, the popularity of Ethernet-based local area networks (LANs) has increased tremendously. In the 1980s, the Institute of Electrical and Electronic Engineers (IEEE) developed an Ethernet standard designated as IEEE 802.3, which was universally adopted by the network industry. However, this standardized Ethernet communication scheme suffered from physical location restrictions since communication devices operating in accordance with IEEE 802.3 needed to be in close proximity to an Alternating Current (AC) power outlet.
Recently, a revised standard entitled “Data Terminal Equipment (DTE) Power Via Media Dependent Interface” (IEEE 802.3af, 2001), was adopted. In accordance with the revised standard, power may be supplied over Ethernet cabling from power supply equipment (referred to as a “PSE”) to an IEEE 802.3af compliant powered device (referred to as a “PD”) when Power-over-Ethernet (PoE) circuitry is deployed within the PSE.
More specifically, in accordance with IEEE 802.3af, forty-eight (48) volts DC is supplied over two out of four available pairs on a CAT-3 or CAT-5 cable with a maximum current of about 350 milliamperes (mA) for a maximum load power of 15.4 watts (W). In some configurations, only a minimum of 12.95 W is available after considering voltage losses over the communication media.
One problem that still remains with the IEEE 802.3af standard is the lack of any mechanism to detect insufficient power supply levels, and upon detection, to automatically alter the functionality of the powered device. The existence of this problem is based on the fact that, in some cases, powered devices (PDs) operate in a mixed environment, where PDs may be powered by PSEs with different electrical characteristics and may further experience different levels of power loss based on the type and length of Ethernet cabling used. Hence, depending on the particular power characteristics of the PSE and the type/length of cabling, a powered device may be powered up and become fully operational in some environments or may be unable to be powered up in other environments.